1. Field of the Invention
The present invention relates to flow control valves generally and more specifically to flow control valves suitable for use in downhole steam generators whose actions snap either to a fully opened position or to a fully closed position.
2. Description of the Prior Art
As a result of past oil shortages, interest has been directed towards the development of downhole steam generator systems for effecting steam simulation in domestic oil reservoirs. Such systems include for example the one set forth in U.S. Pat. No. 4,336,839 to Wagner et al.
In most downhole steam generator systems, a pump at ground-level pumps fuel to the wellhead and down a stringer to the downhole steam generator unit. Prior to a firing of the steam generator, the entire stringer is usually charged with fuel, creating a static column of fluid of 2,500 feet or more. A check valve provided at the base of the stringer serves to prevent fuel flow into the combustor until the proper moment. As the combustor is to be fired, the pump at the wellhead begins applying an overpressure which, in prior art systems, causes the check valve to open and discharge fuel into the combustor.
However, prior art check valves have proven to be very unsatisfactory in downhole steam generators for several interrelating reasons. First of all, such valves are designed primarily to prevent reverse flow, which means that they are designed to have reseating pressures which are characteristically close to their cracking pressures. Consequently, they tend to modulate between their fully opened and closed positions in response to fluid oscillations in the column of fluid passing therethrough. At the intermediate positions, certain elements of these valves are prone to chatter, and chatter is very detrimental to the service life and/or sealing qualities of the valve. These considerations are of special concern in downhole steam generators, because the improper functioning of a valve or its early demise can mean that the whole downhole steam generator unit must be pulled from the well for servicing, a costly task for the operator. Of almost equal concern is that a modulating valve will cause cyclical variations in the rate at which fuel is discharged into the combustor, which effect can ruin combustor performance.
Moreover, the working environment of the downhole steam generator is extremely dirty and filled with caustic gases and fluids. Thusly, any valves used therein must be capable of handling substantial particulate contamination without jamming or the like. Check valves have proven unsatisfactory in meeting these additional demands.
Another problem attendant with the use of check valves in downhole steam generator systems is that the pressure drop across the typical check valve rises in proportion to the flowrate therethrough. Consequently, the ground-level pump must overcome more and more pressure loss as it provides increasingly greater flowrates to the combustor. This additional demand on the pump is undesirable for reasons which are obvious.